How to get the OAuth token to be assigned to my HttpClient using .Net Core 2.2 AddHttpClient?

Question

I'm writing an Asp.Net Core 2.2 application in which I'm adding HttpClient.

        services.AddAuthentication(o => {
            o.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
            o.DefaultAuthenticateScheme = CookieAuthenticationDefaults.AuthenticationScheme;
            o.DefaultChallengeScheme = CookieAuthenticationDefaults.AuthenticationScheme;
        })
            .AddQuickbooksOnline(o => {
                o.ClientId = Configuration["ecm.qbo.client-id"];
                o.ClientSecret = Configuration["ecm.qbo.client-secret"];
                o.Scope.Add(QuickbooksOnlineScopes.Accounting);
            });

        services.AddHttpClient("qbo", c => {
            c.BaseAddress = new Uri("https://sandbox-quickbooks.api.intuit.com");
            c.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            c.DefaultRequestHeaders.Add("content-type", "application/json");
            c.DefaultRequestHeaders.Authorization=// How do I get the token to be assigned here?
        });

        service.AddScoped<IInventoryService, QboInventoryService>();

So, basically I wrote myself an extension which allows me to AddQuickbooksOnline authentication using minimal configuration as many configurations is made by default by the extension.

Once registered and authenticated to my app, I display a GetProducts button to the authenticated user.

I want this button to call upon my ProductController.

public class ProductController : ControllerBase {
    private readonly IInventoryService _inventoryService;
    public ProductController(IInventoryService inventoryService) {
        _inventoryService=inventoryService??throw new ArgumentNullException(nameof(inventoryService));
    }

    public async Task<IEnumerable<Product>> Get() {
        var products=await _inventoryService.GetAllProducts();
        return products;
    }
}

And the service actually depends on the configured HttpClient I want to add in my services configuration.

public class QboInventoryService : IInventoryService {
    private readonly IHttpClientFactory _clientFactory;
    private readonly IConfiguration _config;
    public QboInventoryService(IHttpClientFactory clientFactory, IConfiguration config) {
        _clientFactory=clientFactory??throw new ArgumentNullException(nameof(clientFactory));
        _config=config??throw new ArgumentNullException(nameof(config));
    }

    public async Task<IEnumerable<Product>> GetAllProducts() {
        IEnumerable<Product> products=new List<Product>();

        var client=_clientFactory.CreateClient("qbo");
        var response=await client.GetAsync(config["qbo.products.query"]);

        if(response.IsSuccessStatusCode) {
            var content=await response.Content.ReadAsStringAsync();
            products=JsonConvert.DeserializeObject<IEnumerable<Product>>(content);
        }

        return products;
    }
}

So, in order to make my call to the QBO WebAPI, I need to provide the OAuth token of authenticated user to the HttpClient being instantiated.